Manufacture of phosphates



Nov. 19, 1935.

L. PREISMAN MANUFACTURE OF PHQS PHATE S Filed Oct. 26. 1931 l N V E N TO R ,Zay/3 Pre/swan BY 7W ATTORNEY Patented Nov. 19, 1935 UNITED STATES PATENT FFIQE MANUFACTURE FA PHOSPHATES Louis reisman, Wilmington, Del., assignor to General Chemical Company, New York, N. Y., a corporation of New York Application October 26,1931, Serial No. 571,275

9 Claims.

.recovered alkali metals in the production of alkali metal phosphates. In its more specific aspects, the invention is directed to the recovery of phosphates and alkali metals from the a1- kali metal phosphate mother liquors resulting from the production of alkali metal phosphates by furnacing phosphoric acid and alkali metal sulfate with reducing agents, for example as in the processes of U. S. Patents Nos. 744,128 and 1,037,837 to E. H. Strickler, and the improvements in such processes as disclosed in U. S. Patents Nos. 1,727,551 and 1,747,588 to W. H. Lohmann.

The mother liquor from the above furnace process contains substantial amounts of tri-alkali metal phosphate contaminated With various impurities, including alkali metal sulfate. The latter impurity tends to contaminatethe alkali metal phosphate product with sulfate and, if the mother liquor is discarded to eliminate sulfate impurities, a substantial loss of alkali metal phosphate ensues.

Reworking of diand tri-alkali metal phosphate mother liquors as constituents of the furnace mix in the processes `of the Strickler patents has been proposed, for example, in Levermore U. S. Patent No. 1,866,657, July 12, 1932. In accordance therewith, it has been suggested to rework tri-alkali metal phosphate mother liquors by returning the same to the digest tanks in which the furnace mix is made up. In this Way it has been possible to utilize relatively large `quantities of mother liquor and form a satisfactory furnace mix. However, in order to develop an operating procedure having accurate control of the ratio of alkali metal sulfat-e to phosphoric anhydride in the furnace mix, it has been desirable to convert all the alkali metal present in the mother liquors to alkali metal sulfate by the addition of sulfuric acid. In this manner, considerable quantities of sulfuric acid were consumed in converting the alkali metal in the mother liquors to alkali metal sulfate, the use of such amounts of acid being, of course, an additional item of expense.

It is the principal object of the present invention to provide a method for recovering the phosphate and alkali metal content of alkali phosphate mother liquors, and utilizing the same in the subsequent production of alkali metal phosphates without subjecting the mother liquors to furnacing, thus eliminating the furnacing operation with its attendant difculties, and effecting a saving of the material quantities of sulfuric acid heretofore required in the preparation of the mother liquors for the furnacing operation.

' A further object of the invention lies in the provision of a process in which the alkali metal sulfate content of the mother liquors is eliminated from the system by reaction of the liquor with a soluble alkaline earth phosphate such as mono- 10 calcium phosphate or mono-barium phosphate with resulting production of alkali metal phosphate and alkaline earth sulfate, whereby the sulfate necessary for precipitation of the alkaline earth metal is obtained from the hitherto objectionable alkali metal sulfate of the mother liquor and the alkali metal content of this alkali metal sulfate becomes available in the form of alkali metal phosphate. Another object of the invention is directed to the provision of a process for making alkali metal phosphates in which the a1- kali metal content of the alkali phosphate mother liquors is utilized to partially neutralize phosphoric acid, thus effecting a material reduction in the quantity of alkali, for example, soda ash from outside sources, previously required for this purpose.

For convenience, the invention will be described in connection with the manufacture of sodium phosphates, although the invention is obviously applicable to the production of other alkali metal phosphates.

Briey, the invention comprises the preparation of a phosphoric acid solution, containing soluble calcium as in the form of mono-calcium phosphate, preferably by the digestion of phosphate rock With phosphoric acid containing a small amount of sulphuric acid. The acid solution, after the separation therefrom of calcium sulfate and other undissolved solids, is treated with sodium phosphate mother liquor containing sodium compounds including sodium sulfate, preferably obtained from the processes of the aforementioned patents, in quantities such that the sodium sulfate of the mother liquorreacts with the mono-calcium phosphate content of the acid solution to precipitate substantially all of the sulfate as calcium sulfate, and also such that the mixed solution of mother liquor and phosphoricacid will contain more than of its 50 P205 content as phosphoric acid. The reaction of the sodium sulfate of the mother liquor and the monocalcium phosphate of the acid solution produces principally mono-sodium phosphate and calcium sulfate, and at the same time, the

tri-sodium phosphate and the sodium of the other sodium compounds in the mother liquor react with the phosphoric acid to form monosodium p-hosphate. After settling and separation of the mud containing chiefly calcium sulfate, the resulting clear solution containing mono-sodium phosphate and phosphoric acid is further neutralized by the addition of suitable quantities of soda ash to produce the desired dialkali phosphates.

Referring to the accompanying drawing showing a flow sheet of the improved process, the phosphoric acid solution employed for treatment of the mother liquor is made up, preferably, by digesting in tank I0 phosphate rock With phosphoric acid H3PO4) containing, for example about 15-17% P205 and a small amount of sulfuric acid, so as to produce a phosphoric acid solution containing aboutl to 20% P205 and preferably more than about 3.3% mono-calcium phosphate. A suitable phosphoric acid solution containing the proper amount of mono-calcium phosphate may also be prepared directly by digesting phosphate rock with a deficiency of sulfurie acid. However the acid solution is made up, the calcium sulfate, formed by the reaction of the phosphate rock and phosphoric or sulfuric acid, together With other insoluble impurities are separated from the solution after the digestion by settling and decantation, or by other suitable means in a separator II. While the acid solution collecting in tank I2 should, as noted, contain preferably more than 3.3% monocalcium phosphate, acid solutions containing less than 3.3% may be utilized.

The tri-sodium phosphate mother liquors obtained in the practice of the processes of the above noted patents contain in addition to trisodium phosphate various quantities of other sodium compounds such as sodium sulfate (Na2SO4), sodium carbonate (Na2CO3), sodium aluminate (NaAlO2) sodium hydroxide (NaOH), together with small amounts of organic matter, arsenic and other soluble impurities.

The clear phosphoric acid solution of tank I2 containing mono-calcium phosphate obtained by the digestion of phosphate rock and phosphoric acid is now treated in a vat or tank I3 with trisodium phosphate mother liquor in quantities suflicient so that in the resulting solution, the sulfate of the sodium sulfate ofthe mother liquor reacts With the calcium of the calcium monophosphate of the acid solution and so that the remaining total alkaline sodium compounds in the mother liquor will neutralize the acid solution to such an extent that the mixed solution of mother liquor and phosphoric acid will contain more than 50% of its P205 content as phosphoric acid. The sulfate of the sodium sulfate of the mother liquor and the calcium of the mono-calcium phosphate react to produce calcium sulfate, and the phosphoric acid of the acid solution reacts with the tri-sodium phosphate and other sodium compounds such as sodium carbonate and sodium hydroxide to form mono-sodium phosphate. The purpose of regulating the amount of mother liquor added to the phosphoric acid solution so that the total alkaline sodium compounds in the mother liquor, aside from the neutral sodium sulfate, will neutralize the phosphoric acid solution to such an extent that the mixed solution of mother liquor and phosphoric acid will contain more than 50% of its P205 content as phosphoric acid, the remainder being mono-sodium phosphate, is to prevent precipitation, at this stage, of the so-called white mud, containing phosphates of iron, calcium, aluminum and sodium luosilicate, which precipitation interferes with the settling or filtering out of the calcium sulfate. Although not likely lto occur in practice, should a` situation arise in Which the quantity of the sodium sulfate in the mother liquor is insufficient to provide in the mixed solution of mother liquor and phosphoric acid reacting proportions of sodium sulfate and mono-calcium phosphate, but the remaining alkaline sodium compounds of the mother liquor are present in quantities great enough to neutralize the acid solution to such an extent that the mixed solution of mother liquor and phosphoric acid will contain less than 50% of its P205 content as phosphoric acid, the proper balance in the mixed solution of mother liquor and phosphoric acid may be obtained by the addition thereto of a suitable amount of phosphoric f acid containing no mono-calcium phosphate. For the reason noted above in connection with the prevention of precipitation of the white muds, the mixed solution of mother liquor and phosphoric acid should contain more than 50% of its P205 content as phosphoric acid. On completion of the reactions resulting from the addition of the mother liquor to the phosphoric acid solution, the vliquor containing, for example,

about 0.9 to 1.0% soluble S03 and about 0.3 to

0.5% CaO is ltered or settled in a separator I4 to effect removal of the calcium sulfate and other insoluble impurities. There is no interference with the settling or filtering of calcium sulfate and other insoluble impurities in the absence of white muds.

Indications are that the reactions between the phosphoric acid solution and the mother liquor seem to take place as follows:

On the addition of the mother liquor to the acid solution, from which the calcium sulfate and other insolubles have been separated, the reactions between the tri-sodium phosphate, sodium hydroxide and sodium carbonate appear to be as indicated producing respectively mono-sodium phosphate. According to (2), it will be seen that the sodium sulfate of the mother liquor precipitates the calcium of the mono-calcium phosphate as calcium sulfate, the sodium sulfate thus acting to replace the sulfuric acid formerly -used in the precipitation of the calcium of the calcium phosphate. Considering the reaction of the mother liquor as a whole with the phosphoric acid solution, it will be seen primarily that the sodium contained in the mother liquor acts t0 produce mono-sodium phosphate, and the sulfate of the sodium sulfate operates to precipitate the calcium of the mono-calcium phosphate as calcium sulfate. The important features of the reaction are that the sodium content of the mother liquor is utilized directly to partially neutralize phosphoric acid with the twofold advantage that no sulfuric acid is necessary to first convert the sodium compounds to the sulfate, as when furnacing the mother liquor, and that because of the use of the sodium content to partially neutralize phosphoric acid, the quantities of soda ash necessary for subsequent neutralization to obtain the desired ultimate di-alkali product are greatly reduced. Additionally, since the sulfate of the sodium sulfate of the mother liquor eifects precipitation of the calcium of the calcium mono-phosphate, this eliminates the necessity of employing sulfuric acid to bring about the precipitation of the calcium as has been practiced.

On completion of the reaction between the several constituents in the mother liquor and the phosphoric acid solution containing calcium phosphate, the liquor is allowed to se-ttle or is ltered to effect separation of the calcium sulfate and other insoluble impurities from the solution now containing a mixture of phosphates. The clear solution is then further neutralized in tank l 5 with soda ash in the usual manner to produce the desired di-alkali phosphate. The neutralization of the clear solution with soda ash precipitates as white mud not only the remaining impurities present in the phosphoric acid, but also the impurities originally present in the trisodium phosphate mother liquor, such as sodium aluminate and other soluble metals. The white mud may be separated by filter i6 from the disodium phosphate liquor. The process thus puriies the trisodium phosphate mother liquor, and utilizes the sodium content of the mother liquor as a substitute for a part oi the soda ash required to neutralize the phosphoric acid.

In the event that the arsenic and organic impurities in the mother liquor rise above permissible limits, as indicated by the iinal products, a portion of the mother liquor may be reworked by subjecting the same to the furnacing operation of the Strickler patents. Such furnacing operation eliminates arsenic and organic matter, and thus reduces the quantity of these impurities in the mother liquor circuit to below the permissible amounts.

It will thus be seen that the invention provides a process for the treatment of mother liquors containing tri-sodium phosphate and sodium compounds by which the phosphates and sodium of the liquors may be recovered and the sodium utilized for the neutralization of phosphoric acid without the necessity of subjecting the entire bulk of the mother liquor to a furnacing operation. As to the major portion of the mother liquor, the furnacing operation is eliminated. The invention makes provision for the economic utilization of large quantities of alkali metal phosphate mother liquor without furnacing'. The comparatively large amounts of sulfuric acid heretofore employed in making up the furnace mix to utilize the mother liquors are saved. I'he sodium content of the mother liquor is employed to partially neutralize the phosphoric acid, thus reducing to a large degree the amount of soda ash subsequently required to effect the ultimate neutralization of the acid to the desired extent. Further, the sulfate ci the sodium sulfate of the mother liquor is utilized to eiiect precipitation of the calcium of the calcium phosphate of the acid solution in this way replacing the sulfuric acid heretofore used for this purpose.

The process is not limited to the treatment of trisodium phosphate mother liquors produced in the processes of the Strickler patents. The invention is applicable to the treatment of mother liquors resulting from the manufacture of alkali metal phosphates where such liquors contain sodium in combinations such for example as tri-sodium or di-sodium phosphate from which the sodium may be utilized for the neutralization of phosphoric acid.

In the appended claims, the term neutralized, unless otherwise modified, is intended to denne any degree oi neutralization of a phosphoric acid solution, and is not limited to indicate complete neutralization.

I claim:

1. In the manufacture of alkali metal phosphates involving the iurnacing of a mix containing a phosphate radical and an alkali metal sulfate in the presence of a reducing agent with the production from the furnaced product of alkali metal phosphate and mother liquor containing alkali metal compounds including an alkali metal sulfate, the method of recovering the alkali metal content of the mother liquor which comprises treating the mother liquor with a phosphoric acid solution containing soluble calcium phosphate whereby the sulfate of the alkali sulfate is removed from solution and phosphoric acid is neutralized.

2. In the manufacture of alkali metal phosphates involving the furnacing of a mix containing a phosphate radical and an alkali metal sul.- fate in the presence of a reducing agent with the production from the furnaced product of alkali metal phosphate and mother liquor con-` phosphoric acid solution containing mono-cal-- cium phosphate whereby the sulfate of the alkali metal sulfate is precipitated and phosphoric acid is partially neutralized, separating solid res-v idues from the solution and further neutralizing the solution by the addition of an alkali metal thereto. y

3. In the manufacture of sodium phosphates involving the furnacing of a mix containing a phosphate radical and sodium sulfate in the presence of a reducing agent with the production from the furnaced product of a sodium phosphate and mother liquor containing sodium compounds including sodium sulfate, the method of recovering the sodium content of the mother liquor which comprises treating the mother liquor with a phosphoric acid solution containing mono-calcium phosphate whereby the sulfate of the sodium sulfate is removed from solution, and the sodium content of the liquor is utilized to neutralize phosphoric acid.

4. In the manufacture of sodium phosphates involving the furnacing of a mix containing a phosphate radical and sodium sulfate in the presence of a reducing agent with the production from the furnaced product of a sodium phosphate and mother liquor containing sodium compounds including sodium sulfate, the method of recovering the sodium content of the mother liquor which comprises treating the mother liquor with a phosphoric acid solution containing more than about 3.3% mono-calcium phosphate whereby the sulfate of the sodium sulfate is precipitated as calcium sulfate and the phosphoric acid partially neutralized, separating solid residues from the resulting solution, and further neutralizing the solution by the addition of sodium carbonate thereto.

5. The method of producing sodium phosphate which comprises forming a phosphoric acid solution containing monocalcium phosphate, treating the solution with such quantities of sodium phosphate mother liquor containing sodium com-- pounds including sodium sulfate that the total alkaline sodium compounds in the mother liquor will neutralize the acid solution to such an extent that the mixed solution of mother liquor and phosphoric acid will contain more than of its P205 content as phosphoric acid, whereby calcium sulfate is precipitated and sodium phosphate is produced.

6. The method of producing sodium phosphate which comprises forming a phosphoric acid solution containing monocalcium phosphate, treating the solution with such quantities of sodium phosphate mother liquor containing sodium compounds including sodium sulfate that the sodium sulfate of the mother liquor and the mono-calcium phosphate content of the acid solution are present in reacting proportions and that the total alkaline sodium compounds in the mother liquor will neutralize the acid solution to such an extent that the mixed solution of mother liquor and phosphoric acid will contain more than 50% of its P205 content as phosphoric acid, whereby calcium sulfate is precipitated and sodium phosphate is produced.

'7. The method of producing sodium phosphates which comprises forming a phosphoric acid solution containing more than about 3.3% mono-calcium phosphate, treating the solution with such quantities of tri-sodium phosphate mother liquor containing sodium compounds including sodium sulfate that the sodium sulfate of the mother liquor and the mono-calcium phosphate are present in reacting proportions and that the total alkaline sodium compounds in the mother liquor Will neutralize the acid solution to such an extent that the mixed solution of mother liquor and phosphoric acid will contain more than 50% of its P205 content as phosphoric acid, whereby the sulfate of the sodium sulfate is precipitated as calcium sulfate and the phosphoric acid partially neutralized, separating solid residues from the resulting solution, and further neutralizing the solution by the addition vof sodium carbonate thereto.

8. The method for producing alkali metal phosphate which comprises forming a phosphoric acid solution containing mono-calcium phosphate, treating the solution With such quantities of alkali metal phosphate mother liquor containing alkali metal compounds includingv alkali metal sulfate that the total alkaline metal compounds in the mother liquor will neutralize the acid solution to such an extent that the mixed solution of mother liquor and phosphoric acid will contain more than 50% of its P205 content as phosphoric acid, whereby calcium sulfate is precipitated and alkali metal phosphate is produced.

9. The method for producing alkali metal phosphate which comprises forming a phosphoric acid solution containing mono-calcium phosphate, treating the solution with such quantities of alkali metal phosphate mother liquor containing the alkali metal compounds including alkali metal sulfate that the alkali metal sulfate of the mother liquor and the mono-calcium phosphate content of the acid solution are present in reacting proportions and that the total alkaline alkali metal compounds in the mother liquor will neutralize the acid solution to such an extent that the mixed solution of mother liquor and phosphoric acid will contain more than 50% of its P205 content as phosphoric acid, whereby calcium sulfate is precipitated and alkali metal phosphate is produced.

LOUIS PREISMAN. 

